The present invention relates generally to the field of manually powered vehicles. More specifically, the present invention relates to wheelchairs in which the wheelchair user supplies hand power to propel the chair.
In a typical manually powered wheelchair, the seated chair occupant grasps large wheels on the chair and rotates the wheels in the desired direction of chair movement Circular hand grips are usually attached to the outside of the large wheels to facilitate the procedure. This procedure requires a downward and forward arm movement that tends to lift and push the operator against the back of the seat preventing the operator's full arm strength from being transferred to the wheels.
Manually self-propelled wheelchairs have typically required the use of large wheels to provide the mechanical advantage necessary to convert occupant power to propulsion and to help move the chair over irregular surfaces. The center of gravity of an occupied wheelchair having such large wheels is high and forward of the chair's rear pivotal axis about the large wheels. The downward and forward forces required in manually rotating the wheels tend to move the center of gravity of the occupied chair higher and in a direction that would place the center of gravity behind the pivotal axis thus making the chair less stable. When the self powered movement is up a sloping surface or over abrupt surface obstructions, the center of gravity of the occupied chair can easily move behind the pivotal axis of the chair causing it to tip backwards. The result, again, is that maximum available manual power cannot be effectively transmitted to the wheels when advancing the chair up a slope or attempting to roll over large surface obstructions.